Sea-Level Rise

Welcome to the Division of Coastal Management’s sea-level rise homepage. The information provided here is intended to provide you with information about the state of the science of sea-level rise as well as current events in North Carolina and elsewhere. Those in search of more technical or detailed information can find some on this page, plus links to additional resources. Thank you for visiting, and contact us with any questions or feedback.

Everyone who has been to the beach is familiar with high and low tide. The level of the sea changes over the course of a tidal cycle – about 12 hours. But does the level of the high and low tide stay constant day after day, year after year? No, partly because of natural cycles that raise or lower the level of the ocean, and partly the world has been in a warming phase since the end of the last ice age. The warmer the earth, the higher sea levels will be.

To better understand sea-level rise, it is important first to understand what sea level is, as well as some of the other terms used in talking about sea level and sea-level changes. Put simply, sea level is the average height of the ocean, which is determined by the amount of water in the ocean and how much space it takes up. This is also referred to as eustatic sea level.Sea-level rise is an increase in eustatic sea level that is sustained over a long period of time. Sea-level rise can occur as a result of water or ice being added to the ocean, or by the water in the ocean heating up and expanding to take up more space.

Image credit: Casey Dziuba

Relative sea level is the average height of the ocean in relation to the shoreline. Relative sea-level rise is an increase in the height of the sea in relation to its position along the shoreline. Relative sea-level rise can be caused by an increase in eustatic sea level, subsidence (sinking) of the land, or a combination of both. For example, if eustatic sea level rises by one foot and the land subsides by one foot, that produces two feet of relative sea-level rise.

Sea level has varied enormously in the history of the earth, sometimes rising, sometimes falling, and sometimes staying fairly stable. During the ice ages, water was frozen in giant ice sheets that sat over land, making sea levels low. During the warm interglacial periods, much of the land ice melts and flows into the ocean, raising sea levels around the world. The warm periods also produce thermal expansion of the oceans because warmer water takes up more space than colder water. Thermal expansion is one of the major contributors to sea-level rise at the present time.

Sea-level rise is a very slow and gradual process, and it takes years before its impacts become evident. According to a report by the Intergovernmental Panel on Climate Change (IPCC), global eustatic sea level has risen more than half a foot (0.2 meters or 0.66 feet) over just the past one hundred years.

North Carolina’s shoreline during the ice ages has been as far eastward as the edge of the continental shelf, and during the interglacial periods been almost as far inland as the U.S. Highway 17 corridor. The graphic below shows the approximate position of the North Carolina shoreline about 125,000 years ago when sea level was six to seven meters higher than it is today.

What causes sea-level rise?In the history of the world, sea level has been hundreds of feet higher and lower than it is today due to the natural climate cycle of ice ages and interglacial periods. Changes in sea level can happen due to many factors, including changes in the amount of ice and snow stored on land in the form of ice sheets and glaciers, the local shoreline moving up or down, or through larger scale processes such as global climate change. For more information please refer to the North Carolina Sea-Level Rise Assessment Report.

There is a long history of natural climate cycles that occur on our planet. These climate cycles, which we know about because of the fossil record and ice cores, produce warm and cool periods on earth. Sea level changes are a part of these natural warming and cooling cycles. As the planet gets warmer, seawater expands, glaciers and ice caps melt, and sea level rises. As the planet cools, such as during an ice age, more water becomes taken up as ice on land and sea level falls. It is important to distinguish between weather and climate. Weather refers to conditions over a short period of time such as minutes, days, or weeks. Climate refers to average weather conditions for a particular area over many years, usually decades or longer.

Why do predictions vary?Though scientists generally agree that sea-level rise is occurring now, there is some debate over just how much sea level is expected to rise by the end of the century and beyond. To determine the extent of future sea-level rise, scientists use models which examine the factors, such as greenhouse gas emission scenarios, global average temperature changes, thermal expansion, and melting polar ice caps that will contribute to an increase in sea level.

Scientists also use models to predict how past trends might continue into the future. Because some of the factors cannot be accurately predicted, scientists often use a scenario approach to describe what could happen in the future. The IPCC estimated in a 2013 report that sea level will rise between 10.2 inches (0.26 meters) and 38.6 inches (0.98 meters) by 2100, depending in part on future greenhouse gas emission scenarios and the effect of greenhouse gas concentrations on global temperatures and thermal expansion.

Rate of sea-level rise in North CarolinaSea level has changed a lot in North Carolina’s history. Very ancient differences in sea level can be seen in the geologic and fossil record. Today, fossils of marine organisms, such as fossilized shark teeth, can be found many miles inland from the ocean. This provides evidence that the shoreline was at one time much further inland than it currently is today. More recent changes in sea level are measured using technology such as satellites and tide gauges, which report the sea level at regular intervals. The information provided by tide gauges in North Carolina show that a gradual rise in sea level has been occurring consistently along our state's coast over the last 30 years or more that the gauges have been installed.

Reconstructed and measured sea level in North and South Carolina. Graphic: Kemp et al., 2009.

Tide gauge measurements reveal that relative sea-level rise is higher in the northern coastal plain (north of Cape Lookout) than in the southern coastal plain. This is partly because the northern coastal plain has a higher subsidence rate than the southern plain.

How will sea-level rise affect us?Sea level rise is expected to affect our coast in a number of ways and these effects may range from minor to severe. The following are some examples of things expected to be impacted by sea level rise in North Carolina:

Beaches

Historic landmarks and recreation areas, such as lighthouses and state parks

Public trust resources, such as wetlands and fisheries

Water supplies and water quality

Private property, including land and structures

Barrier islands

Public infrastructure, such as roads and sewer systems

Wildlife habitat

Public lands, such as the Cape Hatteras National Seashore and the Alligator River National Wildlife Refuge

Low-lying coastal areas and barrier islands such as the northern coastal plain are considered extremely vulnerable to sea-level rise impacts. However, it is important to keep in mind that sea-level rise is a long term and very gradual process. While there is no cause for immediate alarm, it is still very important to plan for the future. Some of the impacts of sea-level rise are discussed in more detail below.

Environmental impactsAside from causing property damage, sea-level rise can also have serious impacts on the local environment. Coastal ecosystems, such as estuaries and marshes, will feel the greatest impact of sea-level rise. Estuarine environments, which occur where freshwater and seawater meet, are highly productive ecosystems that help maintain good water quality by acting as a filter for particles and pollutants. The approximately 12,000 miles of estuarine shoreline along the North Carolina coast are also important fisheries habitats. In fact, three out of four commercial fish in the United States use estuarine habitats at some point within their life cycle. Disruption of these important habitats by sea-level rise may result in negative impacts on fisheries and depleted water quality.

Rising sea level will cause saltwater to enter into coastal aquifers, a phenomenon known as saltwater intrusion. Aquifers, which are like large underground lakes, are important sources of drinking water. With saltwater intrusion, the water in the aquifer becomes contaminated with salt and turns undrinkable. Saltwater intrusion is a very serious problem because it threatens the availability of drinking water and can make soils too salty for native plants to grow, creating problems for coastal forests and agriculture. Saltwater intrusion is one of the first impacts that the coast is expected to face with an increase in sea level.

What can I do to be prepared for sea-level rise?Sea-level rise has been described by the Intergovernmental Panel on Climate Change as “inevitable.” However, that doesn’t mean that nothing can be done about it. One of the most important things the average person can do to prepare for sea-level rise is to educate themselves. A number of links on the bottom of this page will direct you to other websites that will provide more in depth information about sea-level rise. Another important thing coastal residents can do is to get involved with their local communities and talk to their local governments about what is being done to prepare their community for sea-level rise. Lastly, every person can do their part to cut greenhouse gas emissions by reducing energy consumption through actions like driving less and using less electricity.